The Human Rights Campaign, the country’s largest LGBTQ rights advocacy organization, is opposing Sen. Susan Collins’s reelection bid in Maine and endorsing her Democratic opponent, Sara Gideon. It’s the first time that the HRC has opposed Collins, a key Republican vote on LGBTQ rights, for reelection.

The endorsement comes a day after Gideon, speaker of the state House, won Maine’s Democratic Senate primary.

“We are fighting for our lives and the only way to advance LGBTQ equality through the United States Senate is to install a new pro-equality majority leader and replace Mitch McConnell,” HRC President Alphonso David said in a statement. “Despite Susan Collins’ record of support on certain key LGBTQ issues, her support of Mitch McConnell and Donald Trump’s agenda, endorsement of Brett M. Kavanaugh’s appointment to the Supreme Court and failure to hold Donald Trump accountable, is simply untenable.”

The HRC’s move is a blow to Collins, who has frequently broken with her party to support LGBTQ priorities in her 24-year Senate career. She’s the only Republican co-sponsor of the Equality Act, legislation to ban discrimination based on sexual orientation and gender identity that was passed by the House but not taken up by the Senate. She was also one of a handful of Republicans — most of them no longer in the Senate — who voted to end the military’s “don’t ask, don’t tell” policy. When the HRC backed Collins six years ago, it cited her “pivotal role in advancing support for LGBT equality” and ability to work across party lines.

That was before Collins’s vote to confirm Brett M. Kavanaugh to the Supreme Court. The HRC condemned that vote, which Gideon cited as the reason she got into the race and which replaced frequently pro-LGBTQ Justice Anthony M. Kennedy with a social conservative. Last month, when a Supreme Court majority voted that the 1964 Civil Rights Act prevents LGBTQ employees from discrimination, Kavanaugh dissented.

Before today, the HRC had gotten some criticism for not endorsing against Collins. Two other influential liberal groups, Planned Parenthood and the League of Conservation Voters, had already endorsed Gideon. But the HRC held back, in part, because Gideon’s more liberal primary opponents identified as bisexual and non-binary. The pro-LGBTQ Victory Fund had endorsed another Gideon opponent; in March, that candidate ended his bid and supported the front-runner.

Collins, elected in 1996, won reelection easily in 2002, 2008 and 2014. But polls since the Kavanaugh vote have found Democrats who had supported her for years souring on their moderate Republican senator. According to campaign filings from late last month, Gideon has raised $22.8 million, while Collins has raised $16.1 million. Gideon ended the primary with slightly more cash on hand, and opponents of Collins’s Kavanaugh vote had already raised $3.7 million in a separate fund, to be given to whoever won Tuesday’s Democratic primary.

Gideon is one of several Senate candidates, all Democrats, who will receive the HRC’s support Wednesday. They include Colorado’s John Hickenlooper, Iowa’s Theresa Greenfield, Georgia’s Jon Ossoff and Texas’s MJ Hegar, as well as Massachusetts Sen. Edward J. Markey, who is facing Rep. Joe Kennedy in a September primary.